The present invention relates to organic electroluminescent devices. More in particular, the present invention relates to electroluminescent devices for converting an energy of an electric field applied directly to a light energy and capable of producing surface luminescence for a large area different from conventional incandescent lamps, fluorescent lamps or light emitting diodes.
Electroluminescent devices are grouped into the following two categories depending on the difference of the luminescent excitation system:
(1) an AC driving electroluminescent device in which luminescent material is excited by the local transfer of electrons or holes in a luminescent layer and emits light only by an AC electric field, and PA1 (2) a DC driving electroluminescent device in which luminescent material is excited by the injection of electrons and holes from an electrode and this re-combination in a luminescent layer and actuated only by a DC electric field. PA1 (2) a condensated polycyclic hydrocarbon such as naphthalene, anthracene, naphthacene, phenanthrene, a substituted or unsubstituted indene, indenone, naphthocyclobutene, a substituted or unsubstituted benzocyclohexene, naphthocyclohexene, etc.; PA1 (3) an oxygen heterocycle such as a substituted or unsubstituted furane, benzofuran, isobenzofuran, pyrrone, isonaphthofuran, etc.; PA1 (4) a sulfur heterocycle such as thiophene, isobenzothiophene and isonaphthothiophene, etc.; PA1 (5) a nitrogen heterocycle such as substituted or unsubstituted pyrrole, imidazole, pyridine, pyridazine, pyrazine, pyrimidine, quinoline, quinazoline, triazine, pyrazole, isoindole, etc.; PA1 m is an integer of 1 to 6; and PA1 n is an integer of 1 to 6. PA1 X.sup.6 is H, CH.sub.3, C.sub.2 H.sub.5 or ##STR7## and .kappa. is 0 or 1) ##STR8## (in which X.sup.7 is H or O) ##STR9## (in which X.sup.8 is O or C(CN).sub.2) ##STR10## (in which X.sup.9 is H or CH.sub.3) ##STR11## (in which X.sup.10 is O or C(CN).sub.2) ##STR12## (in which D is O, S or N, Y.sup.1 is H, and PA1 i is 0 or 1) ##STR13## (in which E is O or S) ##STR14## (in which E is O or S) ##STR15## (in which G is S or NH) ##STR16## (in which Z represents: (1) substituted or unsubstituted (C.sub.4 -C.sub.6) cycloalkenes containing at least one double bond in the ring, PA1 (2) (C.sub.4 -C.sub.6) cycloalkenes having an aromatic ring and containing one double bond in the ring, PA1 (3) 5- or 9-membered heterocycles containing a double bond and at least one oxygen in the ring, PA1 (4) 5-membered heterocycles having an aromatic ring, and containing one oxygen and at least one double bond in the ring, PA1 (5) 6- or 7-membered heterocycles having C.sub.4 or C.sub.6 cycloalkene with at least one double bond, and containing two oxygen atoms in the ring, PA1 (6) C.sub.5, C.sub.6 or C.sub.11 heterocycles containing 1 or 2 sulfur atoms in the ring, PA1 (7) 5- or 6-membered heterocycles containing 2 to 4 nitrogen atoms and at least one double bond in the ring, PA1 (8) 5-, 6- or 8-membered heterocycles having an aromatic ring, and containing at least one double bond and 1 or 2 nitrogen atoms in the ring, PA1 (9) 5- or 6-membered heterocycles containing 1 or 2 nitrogen atoms and, optionally, oxygen atom in the ring; PA1 Ar represents ##STR17## m represents an integer of 1 to 6; and n represents an integer of 1 to 6.
The AC driving electroluminescent device generally uses, as a luminescent material, an inorganic compound comprising ZnS and Mn, Cu, etc. added thereto, but it involves drawbacks of requiring high AC electric field of higher than 200 V for the driving, much production cost, and insufficient luminance and durability.
On the other hand, the DC driving electroluminescent device uses, as a luminescent layer, a thin film comprising an organic compound, and it has high power conversion efficiency and high luminance.
For instance, Japanese Patent Application Laid-Open (KOKAI) No. 59-194393 (U.S. Pat. No. 4,539,507) discloses an electroluminescent device comprising an anode, a hole transporting zone, an organic luminescent zone and a cathode, in which the total film-thickness for the anode, the hole transporting zone and the organic luminescent zone is less than 1 .mu.m, and at least one of the anode and the cathode allows to permeate at least 80% of radiation rays at a wavelength of not less than 400 nm and has at least 9.times.10.sup.-5 W/W of power conversion efficiency. As the fluorescent whitening agent of the electron transporting compound forming the organic luminescent zone, there are used, 2,5-bis(5,7-di-t-benzyl-2-benzoxazolyl)-1,3,4-thiadiazole; 4,4'-bis(5,7-t-benzyl-3-benzoxazolyl)stilbene; 2,5-bis(5,7-di-t-benzyl-2-benzoxazolyl)thiophene; 2,2'-(p-phenylenedivinylene)-bis-benzothiazole; 4,4'-bis(2-benzoxazolyl)biphenyl; 2,5-bis(5-(.alpha.,.alpha.-dimethylbenzyl)-2-benzoxazolyl)thiophene; 4,4'-bis(5,7-di (2-methyl-2-butyl)-2-benzoxazolyl)stilbene; and 2,5-bis(5,7-di-(2-methyl-2-butyl)-2-benzoxazolyl)-3,4-diphenylthiophene.
U.S. Pat. No. 4,720,432 also discloses an electroluminescent device comprising in sequence, an anode, an organic hole injecting and transporting zone, an organic electron injecting and transporting zone, and a cathode, characterized in that the organic hole injecting and transporting zone is comprised of a layer in contact with the anode containing a hole injection porphyrnic compound and a layer containing a hole transporting aromatic tertiary amine interposed between the hole injecting layer and the electro injecting and transporting zone.
Further an electroluminescent device comprising an anode, a hole transporting organic material layer, a luminescent layer, an electron transporting organic material layer and a cathode and emitting yellow light is reported in Jpn. Journal of Applied Physics, Vol. 27, pages 713-715, and there are exemplified N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine as the hole transporting organic material, 3,4,9,10-perylene tetracarbonic acid bisbenzimidazole as the electron transporting organic material and phthaloperinone as the luminescent material.
However, it can not be said that sufficient study has been made on the DC driving electroluminescent device using organic compounds as the luminescent material, with respect to the material and device-forming technique, and there are many problems with respect to the improvement of luminance, the control of wavelength of the emission light, the durability and the increased working life.
As a result of the present inventors' earnest studies in order to provide an electroluminescent device having high luminance and excellent durability with long lasting luminescent performance, and providing versatile wavelength of emission light, capable of accurately selecting luminescent hues such as blue, green and red while considering the application to full color display, it has been found that an electroluminescent device comprising as a luminescent layer at least one layer of an organic compound sandwiched between an anode and a cathode, in which the said organic compound layer comprises an organic compound represented by the following formula (I'), can be driven at a low applied voltage, can provide an emission light of high luminance for a long period, and can accurately select the luminescent hues. ##STR1## (where B' represents: (1) a cyclic hydrocarbon such as a substituted or unsubstituted cyclobutene, cyclohexene or cyclohexadiene, a substituted or unsubstituted benzene, p-benzoquinone, cyclopentadienone, diphenyl, etc.;
Ar represents benzene, biphenyl, methoxybenzene or naphthalene;
The present invention has been attained based on this finding.